Fingerprint recognition or fingerprint authentication refers to the automated method of verifying a match between two human fingerprints. Fingerprint readers (or modules) are commonly used devices to meet such requirement. Fingerprints are one of many forms of biometrics used to identify individuals and verify their identity. There are two major classes of algorithms (minutia and pattern) and four sensor designs (optical, ultrasonic, passive capacitance, and active capacitance) wildly used in fingerprint readers or modules.
There are two inexpensive fingerprint readers having been developed and grown fast in recent years. One is the optical scanner. It utilizes a Charge-Coupled Devices (CCD) or CMOS Image Sensor (CIS) to digitize fingerprints on the fingertip. The other is the capacitance-measuring type, usually made as semiconductor chips, fingerprint reader. It uses tiny capacitors to sense the fingerprint pattern on the fingertip. Both the two devices mentioned above capture fingerprint images for software algorithms to analyze geometric patterns of the feature points, for example, the ridges and grooves of each finger. Then, a comparison between the captured image and those from the fingerprint of the legal login user can be performed.
All the types of fingerprint reader may suffer the fingerprint residue problem, which may reduce the performance of the fingerprint recognition algorithm, or even endanger security.
A fingerprint reader can be a personal device and used by a single user. For example, a fingerprint reader on a notebook computer replaces the password entry method to initiate operation system by a registered fingerprint image. A fingerprint reader can also be used by a group of people. For instance, a fingerprint reader in an access control system allows entrance of specified staffs of a company by identifying their fingerprints. For the later case, if a user has a greasy finger, the fingerprint residual left on the fingerprint reader will reduce the image quality of the captured fingerprint and then affect the accuracy of the system. More importantly, for the portable fingerprint devices, the fingerprint residual left on the image sensing surface means a high security risk, because the fingerprint image can be recovered from the fingerprint residue with known retrieval techniques.
There are different ways to eliminate residual of fingerprints. For example, please refer to FIG. 1 which discloses an optical fingerprint imaging system of U.S. Pat. No. 7,515,252. It includes an optically transparent platen 1 and a protective film 2 removably attached to the platen 1 with an adhesive 3. The protective film 2 protects the platen 1 from scratches, pits, containment, and other surface damage and is substantially chemically inert to petroleum-based substances which may come into contact when receiving a finger 4 pressed to take a fingerprint image. The adhesive 3 has a selected cohesive strength that allows the protective film 2 and adhesive 3 to be removed from the platen 1 without leaving behind an adhesive residue. The protective film 2 can be replaced periodically as part of maintenance of the platen 1 to allow continued acquisition of accurate fingerprint images. Although residual of fingerprints may be eliminated by the aforementioned method, it requires the transparent platen and the protective film which cause inconvenience in use and extra cost for fingerprint recognition.
Fingerprint residual can also be eliminated by use of lipolytic enzymes. For example, please refer to FIG. 2. It discloses a substrate 5 surface having self-cleaning function by forming an anti-fingerprint film 6 with a porous structure thereon. When the porous structure including a lipolytic enzyme is formed on the surface of the substrate 5, contaminants 7 decomposed by an enzyme are absorbed into a pore, and thus detectable contamination 7 from a surface of the substrate 5 can be removed by the anti-fingerprint film 6. By this way, contamination or fingerprint residuals on the surface of a display device or electronic device can be effectively reduced. However, the porous structure may not be strong enough to bear friction of the finger after many times of use.
Therefore, a portable device having fingerprint recognition function and is free from residual fingerprint is desperately desired.